<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>mask</title>
  <style>
    html,body{
      width: 100%;
      height: 100%;
      padding: 0;
      margin: 0;
    }
    .mask{
      width: 400px;
      margin: auto;
      border: 1px solid red;
    }
    .mask-title{
      height: 50px;
      background: rgb(233, 20, 49);
      display: flex;
      justify-content: space-between;
      align-items: center;
      padding: 0px 20px;
      color: white;
    }
    .mask-body{
      height: 200px;
      padding: 10px 20px;
    }
  </style>
</head>
<body>
  <div class="mask">
    <div class="mask-title">
      <div>这里是标题</div>
      <div>关闭</div>
    </div>
    <div class="mask-body">
      这里是内容....
    </div>
  </div>
</body>
</html>